Boulder, Colo., USA - New geophysical data show that fault slip during the March 2005 magnitude 8.7 (Mw) earthquake off the west coast of northern Sumatra, Indonesia (also referred to as the Simeulue-Nias earthquake), was stopped by the topography on the downgoing plate.
Earthquakes in subduction zones, where one tectonic plate is forced beneath another, usually break only a part of the plate boundary fault. The pieces that break independently are known as segments. Topography on the top of the downgoing plate has often been suggested a cause of this segmentation, but there are few examples where this topography is as well-known as well as the details of earthquake rupture.
Data collected over the subduction zone offshore of Sumatra, Indonesia, has enabled the top of the downgoing plate to be mapped across a long-lived segment boundary at one end of the rupture zone. Seismic reflection data, similar to that used to find oil reserves, gives a detailed image of the shape of the downgoing plate. A 3-km high on the top of the plate over a 15-km by 30-km region matches where the 2005 earthquake rupture stopped. The topographic high appears to strengthen the plate boundary, and only very large earthquakes would break through this barrier.
This survey by Timothy Henstock and colleagues spans a complex segment boundary zone between the southern termination of the Mw 8.7 earthquake and the northern termination of a major 1797 earthquake that was partly filled by a Mw 7.7 event in 1935. They have identified an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. They note that the high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.
Downgoing plate topography stopped rupture in the A.D. 2005 Sumatra earthquake
Timothy J. Henstock et al., National Oceanography Centre Southampton, University of Southampton, European Way, Southampton SO14 3ZH, UK. This article is OPEN ACCESS online at http://dx.doi.org/10.1130/G37258.1.
Other recently posted GEOLOGY articles are highlighted below:
Ocean warming, not acidification, controlled coccolithophore response during past greenhouse climate change
Samantha J. Gibbs et al., Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, European Way, Southampton SO14 3ZH, UK. This article is OPEN ACCESS online at http://dx.doi.org/10.1130/G37273.1.
Current carbon dioxide emissions are an assumed threat to oceanic calcifying plankton (coccolithophores) not just due to rising sea-surface temperatures, but also because of ocean acidification (OA). This assessment is based on single species culture experiments that are now revealing complex, synergistic, and adaptive responses to such environmental change. Despite this complexity, there is still a widespread perception that coccolithophore calcification will be inhibited by OA. These plankton have an excellent fossil record, and so we can test for the impact of OA during geological carbon cycle events, providing the added advantages of exploring entire communities across real-world major climate perturbation and recovery. Here we target fossil coccolithophore groups (holococcoliths and braarudosphaerids) expected to exhibit greatest sensitivity to acidification because of their reliance on extracellular calcification. Across the Paleocene-Eocene Thermal Maximum (56 Ma) rapid warming event, the biogeography and abundance of these extracellular calcifiers shifted dramatically, disappearing entirely from low latitudes to become limited to cooler, lower saturation-state areas. By comparing these range shift data with the environmental parameters from an Earth system model, we show that the principal control on these range retractions was temperature, with survival maintained in high-latitude refugia, despite more adverse ocean chemistry conditions. Deleterious effects of OA were only evidenced when twinned with elevated temperatures.
The crucial role of temperature in high-velocity weakening of faults: Experiments on gouge using host blocks with different thermal conductivities
Lu Yao et al., State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China. This paper is online at http://dx.doi.org/10.1130/G37310.1.
Fault friction controls the generation of earthquakes. When slip occurs during earthquakes, the faults weaken dramatically. Understanding the mechanisms that cause fault weakening is fundamental to understanding earthquake propagation and slip. The proposed weakening mechanisms include thermally and non-thermally activated ones, but their dominance remains enigmatic because it is difficult to control temperature independently in high-speed sliding due to the coupling of frictional heating and experimental conditions (e.g. slip rates and stresses). This study introduces different thermally-conductive driving (host) blocks to sandwich incohesive fault rocks (gouges) in high-speed friction tests, and succeeds in varying temperature conditions by changing the speed of heat conduction out of the tested samples. The experiments show that high-velocity weakening of gouge becomes more pronounced as thermal conductivity of host blocks decreases. Temperature calculation and microstructural observations suggest big differences in the slip-zone temperature in the case of different host blocks. These results demonstrate that temperature rise driven by frictional heating is essential in causing dynamic weakening of fault gouge. The observed results are most likely to be explained by the flash-heating model that considers the role of the slip-zone temperature. Moreover, the presence of nanoparticles alone is not sufficient to cause dynamic weakening of faults.
Rift to drift transition in the South Atlantic salt basins: A new flavor of oceanic crust
Ian O. Norton et al., Bureau of Economic Geology, University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758, USA. This paper is online at http://dx.doi.org/10.1130/G37265.1.
A long-standing puzzle in understanding of South Atlantic tectonics has been the geometric fit of Africa and South America at the time of deposition of the extensive salt basins found on both margins. The seaward limit of salt has usually been interpreted as the inner limit of oceanic crust (LOC), but resulting LOCs do not match in plate reconstructions for the Aptian, the time of salt deposition. In this paper, we present seismic data from the Brazilian Campos Basin that shows "seaward dipping reflectors" (SDRs) 60 km inboard of the edge of salt. SDRs form at the boundary between continental and oceanic crust, implying that the LOC is actually not at the edge of salt. The same seismic data shows that rift section associated with continental breakup ends at the SDRs, so mapping the limit of rift structures throughout the South American and African salt basins defines new locations of the LOCs. These LOCs match geometrically in an Aptian plate reconstruction. We develop a new model for crustal formation under the salt, suggesting that it is oceanic crust formed intrusively under flowing salt, with normal oceanic crust forming when salt was thin enough for volcanism to break through the salt.
Evidence for cavity-dwelling microbial life in 3.22 Ga tidal deposits
Martin Homann et al., Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany. This article is online at http://dx.doi.org/10.1130/G37272.1.
Shallow-water environments were harsh places for early life because the intense ultraviolet (UV) radiation penetrating the Archean ozone-free atmosphere, the high and strong tides, and the constant threat of desiccation allowed only the survival of the very fittest. Early microorganisms likely found adequate protection against these conditions in caves or cavities in the shallow subsurface; however, the oldest traces of cavity-dwelling life were known only from strata about 2.7 billion years old. Now, Ph.D.-student Martin Homann at the Freie Universität Berlin and his coworkers, while studying well-preserved tidal deposits even 500 million years older, found evidence that ancient microbial communities thrived in cavities on beaches underneath thin sand layers. These layers, a few mm to cm thick, may have been sticky and glued together by early photosynthetic bacteria or their decomposed matter, holding up the roof. Underneath, tiny microstromatolitic columns, made of kerogenous laminae and silica, hung from the roof of these cavities, much as they do today in occasionally flooded sandy shorelines. The composition of the kerogen and the presence of filamentous microfossils also suggest that a rich and well-adapted microbial community inhabited these fluid- and/or air-filled cavities. Early life apparently adjusted quickly to the nutrient-rich but harsh conditions of tidal zones, carving out distinct ecological niches.
Short magmatic residence times of quartz phenocrysts in Patagonian rhyolites associated with Gondwana breakup
Susanne Seitz et al., Institute of Earth Sciences, University of Lausanne, CH-1015 Lausanne, Switzerland. This paper is online at http://dx.doi.org/10.1130/G37232.1.
Knowledge about large volcanic systems has increased substantially in the last decades. Nevertheless, the nature of crustal magma storage, the "trigger" of eruptions and the residence times of crystals in the magma chamber are still topics of discussion. A key parameter in understanding the evolution of magmatic systems is the time scale on which changes occur. Chemically zoned crystals can record various magmatic processes and the time scales for these processes can be extracted using element diffusion as a chronometer. To capitalize on the information contained in mineral zoning, one needs to be able to measure the compositional variations with a spatial resolution that is significantly better than the characteristic diffusion length scale. This is one of the first studies using nanoscale secondary ion mass spectrometry (NanoSIMS) to measure titanium profiles with sub-micrometer spatial resolution in quartz phenocrysts from a Jurassic rhyolite of a silicic large igneous province (Chon Aike Province, Patagonia). The NanoSIMS profiles are used to calculate the time scale (5.6 plus or minus 2.2 years) for quartz crystallization, which is among the shortest documented for large rhyolitic eruptions. Additionally, in combination with the observed growth textures, this study highlights the implication of growth kinetics of quartz crystals for Ti-in-quartz thermometry.
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